SiC (silicon carbide) power semiconductor devices are promising for high voltage, high frequency and high temperature operation, thus enabling higher efficiency and higher power density designs. Among the currently available SiC active switches, the normally-on type (N-on) JFET is the most mature device. However, it has certain features which make its use more challenging compared to standard normally-off type devices [1],[2]. For the operation of N-on JFET components a negative voltage must be applied to turn off the switch and to keep the switch in a blocking state. Further, due to the normally-on characteristic, the switch or a circuit employing the switch requires additional protection circuits and startup circuits.
Startup and protection are exemplary issues which are considered to prevent a short-circuit fault condition when the gate control signal is lost or the auxiliary power supply (APS) is not operating. Auxiliary power supply is used for generating voltages required for controlling the component. In a component that is of a normally-on type, the loss of auxiliary power means that the component is turned on. Similarly, in a startup operation a circuit having normally-on switches may be powered before the auxiliary power supply is able to produce voltages for the control circuit. This may lead to a situation in which current passes through normally-on switches in an uncontrolled manner.
FIG. 1 shows an exemplary back-to-back converter structure based on N-on type JFETs as an example. In FIG. 1, both a startup operation and an auxiliary power supply fault condition are considered.
If the N-on JFETs were used without any additional protection scheme, two possible short-circuit paths are possible, as shown in FIG. 1. A phase-to-phase short-circuit is presented in FIG. 1 both on a grid side and on a motor side. In the phase-to-phase short-circuit, the input phases from the grid are short-circuited as a current path is formed between the phases through the conducting switches. For example, it is shown that the current from the supplying grid passes through switches JL1 and JL2 such that these switches connect different phases directly together.
FIG. 1 also shows a motor side short-circuit that is formed similarly as the grid side short-circuit. The motor side short-circuit connects two motor phases through switches that are conducting.
In a phase leg short-circuit the DC link capacitor is short-circuited through a series connection of switch components. In normal operation the series connected switches should not be controlled conductive at the same time. Once control of the components is missing, the DC link capacitor is short-circuited, causing excessive current in the short-circuit path.
In order to prevent these fault conditions, a cascode structure utilizing a low-voltage MOSFET is a popular protection method, which makes the N-on JFET behave as a normally-off type (N-off) switch [3]-[7]. Therefore, during a fault condition, the converter acts exactly as a diode rectifier when all the gate signals are off. FIG. 2 shows an N-on JFET-based converter employing a direct-driven cascode structure with a startup circuit, i.e. RS-up and SS-up. Although this configuration is promising in terms of protection and startup, the total number of components is very large due to the additional MOSFETs and their gate drivers.
FIG. 2 shows combined driver circuits for both N-on JFET (GDJ) and MOSFETs (GDM). The operation of such a combined driver is such that when auxiliary power is available, the MOSFET is controlled conductive and normal switching operations are carried out with the N-on JFET. Should the auxiliary power from the N-on JFET disappear, the MOSFET is able to control the N-on JFET into a blocking state. Further, if the auxiliary power from the MOSFET disappears, both components are controlled automatically to a blocking state and thereby the short-circuit paths are eliminated.
Reference [8] proposes a simple startup circuit for a grid side converter. The startup circuit turns off the switches automatically at startup by utilizing the voltage of an auxiliary rectifier. However, the auxiliary rectifier is directly connected to the gate driver, which affects the gate driver design and performance. Moreover, the resistance connected between the auxiliary rectifier and the gate drivers may cause extra power losses during normal operation.
To protect the motor side converter, an additional switch has been inserted in the negative rail in [9]. This can prevent short-circuits in phase legs, but the motor side short-circuit is not considered. In [10], a self-protection circuit which monitors the gate-source voltage of the JFET is proposed. However, the additional high voltage linear regulator to provide the negative voltage increases the total circuit complexity.